You might want to consider taking your camera for a full service.  It seems
like a shutter problem. Now I don't know how easy it may be to find another
shutter if this is non repairable.  I have an FA with the MD15 and I
realized that when I shoot in Shutter priority mode there is a chance I will
get burned the way you did.  I had my camera "repaired" but it still does it
every once in a while.  They did not replace the shutter, they only
"cleaned" it.  The remedy: Even when you have no film in there, I shoot a
rip of some 30 shots with the motor drive every couple of weeks if there is
no opportunity to shoot something good.  It helps the shutter from getting
stuck.  Oh if I have film in there, I put on the lens cap, turn it to manual
mode at 1/4000 and the lens at f:1/32 and I keep the double exposure lever
on.  Nothing happens to the film.  It gets exposed to a dark situation, but
since I've been doing that I realized that I have way less frames lost (none
lost for a while actually).  It has to do with the magnets of the shutter
getting worn out due to non functioning.  That reminds me, I need to do
that!
Best of luck

Open the back of your camera and (w/ the MD 12 removed) watch through the
shutter as you go through each exposure speed.  You should see the
difference as you go up the dial.  If you start at the bottom and count
seconds you will be able to judge the intervals at the higher speeds more
easily.

If your shutter operates correctly, then add the MD-12 and repeat.  However,
the only thing the MD12 can really screw up is the advance.  It has no
control over the shutter other than to release it.  So, since your camera
advances and fires, I suspect you will find the problem to be in your
camera.

Don't know about FE2s, I have the old FMs and an F4.  But it would seem if
it were your battery, the camera would either fire or not. Of course, you
could spring for a new battery to check this.  If you haven't, check the
shutter speeds first with he old battery and then use a new one.  If you
notice a difference, try film.  The beauty of the FM is that there is no
electronic control module to factor in; it would *have* to be the shutter
assembly.

You may get lucky and find the shutter just needs to be flushed; or, the
shutter may have been damaged by a hard knock or by a finger poke (it does
sound like the shutter hangs and decreases exposure).  OTOH, it could be the
control module.

In any event, I would send it direct to Nikon for the repair.  You may well
be surprised at the cost of having the camera fixed by them and they try to
have cameras back in 7 days.  Either that or buy the Tomosey book on Nikon
camera service.  The FE2 is a fairly simple camera and getting to the
shutter is not all that hard.  However, what are you going to do if the
shutter assembly or control module is bad?  That is why I suggest sending it
to Nikon.

Sounds like a shutter or metering problem.

If the slide film was fogged it would have gone lighter not darker.

Does the suggested shutter/apeture combination make sense? If the metering
appears correct, I would say it's the shutter.

Not likely. They would have to have two process'. E-6 and C-41  with severe
underdevelopment at the same time.

It seems you have severe underexposure. You do have one mechanical shutter
speed on the FE-2. It is 1/90 of a second. Compare this shutter speed by sound
or looking at the shutter with the electronic shutter speed of  1/90.(Or it's
closest match)

Negative film is going to have a much wider exposure latitude than any
transparency film. This is why you saw an image. After close inspection of your
*totally black* transparency film, you may see some type of image. Although
very very dark.

The FE-2 is wonderful camera, I've had one for years. Good luck!

You mention the temperature variance while you were shooting, what was
the temperature for the rolls that came out unexposed? Batteries will
not last as long, nor work as well in extreme cold conditions (and
-10f qualifies for that).

I own the same combination FE2 & MD12 (and I experienced similar
problems with a NikonEM). The FE2 manual mentions that in very cold
weather, your on-board battery will not provide enough energy to power
the auto-exposure. There is an external battery pack, uses AA
batteries, connects to the camera via a cable and the pack itself then
lives in a pocket next to you and under your jacket. The cable screws
in to the on-board battery cover threads, and replaces the on-board
battery. The batteries stay warm, the camera continues to funtion.

There is a down side to the external battery pack; you can't use your
MD12.

If some of the film under/un-exposed was shot in warmer temperatures
BUT after the session in the cold, it's still possible your on-board
battery hadn't recovered from the cold.

That being said, if the questions about when in sequence your not
properly exposed film was shot in don't hold, then it could be an
electronics problem. Have the problems recurred with a brand-new
battery in place? If not, then it's probably a safe assumption that
the battery didn't cope well with the cold.

Mad Shutter-bug
is located generally in North Central Florida with camera in hand